Predicting a NOx value (to be referred to hereafter as an engine outlet NOx value) of exhaust gas discharged from an internal combustion engine such as a diesel engine is an important element in control of an exhaust gas processing device having a NOx catalyst such as an LNT occlusion reduction catalyst or an SCR selective reduction catalyst. The engine outlet NOx value deteriorates due to various factors such as an operating state of the engine and catalyst deterioration, and therefore, during authentication tests performed in respective countries, the engine outlet NOx value is evaluated by multiplying an initial exhaust gas value by a deterioration coefficient, taking these factors into account. The engine outlet NOx value may be measured precisely by providing a NOx sensor on an upstream side of the exhaust gas processing device so that the engine outlet NOx value is detected directly, but in terms of cost limitations, durability, and the need to apply OBD (failure diagnosis), the NOx sensor is preferably provided on a downstream side of the exhaust gas processing device.
In a case where the NOx value of the exhaust gas is detected by providing the NOx sensor on the downstream side of the exhaust gas processing device, the engine outlet NOx value is predicted by detecting a NOx value (to be referred to hereafter as a catalyst outlet NOx value) of the exhaust gas following passage thereof through the exhaust gas processing device.
This type of exhaust gas purification system, Japanese Patent Application Publication No. 2000-18023, for example, discloses an exhaust gas purification system having a diagnosis function for diagnosing a degree of deterioration in a NOx occlusion catalyst on the basis of detection values of NOx sensors provided on the upstream side and the downstream side of an exhaust gas processing device.